1. Field of the Invention
The present invention relates to a color-image forming apparatus, and relates, more particularly, to a color-image forming apparatus of an electronic photographing system such as a laser printer and a laser-copying machine.
2. Description of the Related Art
Along with the progress of digitalization and coloring of images in recent years, there has been an increasing trend to utilize a color-image forming apparatus, like a laser printer and a laser-copying machine, which expose a photosensitive member by scanning it with laser beams. Particularly, in the case of forming a color image, the four colors black, yellow, magenta and cyan are superimposed. Therefore, this type of color-image forming apparatus has lower productivity than that of the conventional black and white image forming apparatus. In order to solve this problem, a tandem color-image forming apparatus that simultaneously forms four color images has been devised.
The tandem color-image forming apparatus has a plurality of photosensitive members. This color-image forming apparatus exposes each photosensitive member with each laser beam that has been modulated according to image data which has divided image into each of the color component. The color-image forming apparatus then develops these photosensitive members, and superimposes the developed color images on the same transfer medium, to thereby form a color-combined colored image. Therefore, the tandem color-image forming apparatus has a substantially improved productivity.
However, on the other hand, unless image positions of the colors are matched correctly, there occurs a shift in the position of the colors due to variations in the optical characteristics of the laser beams emitted from an exposure apparatus. Thus, it becomes impossible to maintain the quality of a printed image. There are five items that are necessary for the positioning. They are a write start position of a scanning line in a main scanning direction (hereinafter to be referred to as side registration), a write start position of a scanning line in a sub-scanning direction (hereinafter to be referred to as lead registration), a write ending position of the scanning line in the main-scanning direction, or a print width (hereinafter to be referred to as a scale factor), a curve of the scanning line itself (hereinafter to be referred to as a scanning line curve), and an inclination of the scanning line (hereinafter to be referred to as a scanning line inclination). A high-quality image can be formed only when conditions relating to these five items are satisfied.
Further, according to the tandem color-image forming apparatus, the type of the exposure apparatus is broadly classified into two types. FIG. 16 shows an apparatus having four scan exposure devices 300 arranged therein, each device having one laser beam (hereinafter to be referred to as a four-series tandem system). The four-tandem system has four scan rotation motors 302 for the four exposure devices respectively. Therefore, a feature of this system is that independent laser beam scanning positions exist.
An apparatus for scanning four light beams with one motor is disposed in Japanese Patent Application Laid-Open Publication (JP-A) No. 3-142412 (This system will hereinafter be referred to as a spray paint system). As the spray paint system has one scan rotation motor as a deflector, this system is characterized by the laser beam positions being related to each other.
First, the four-series tandem color-image forming apparatus will be described in detail. As shown in FIG. 16, the color-image forming apparatus of the four-series tandem system has four separate scan exposure devices 301. These scan exposure devices 300 expose color images of K (black), Y (yellow), M (magenta), and C (cyan) respectively. Each scan exposure device 300 carries out an image exposure for each color on a photosensitive member 304 provided for each color, to thereby form an electrostatic latent image. After completing the exposure, each electrostatic latent image on each photosensitive member 304 is developed by each developer 306. Each developed image is transferred onto a transfer belt 308 as a single transfer member. At the time of this image transfer, the respective colors are sequentially superimposed to form a color image. This system combines the colors (that is, the positioning of scan beams) by taking the above-mentioned five items into consideration. According to this color-image forming apparatus of the four-series tandem system, it is necessary to provide a special mechanism for controlling the phases of the rotation of the four motors, as these motors operate independently of each other.
On the other hand, according to the color-image forming apparatus of the spray paint system, generally, the scan exposure device itself is relatively compact, as the laser beams corresponding to the respective colors are scanned by one motor. A color image is formed in a similar manner to that of the above-described color-image forming apparatus of the four-series tandem system. However, a feature of the spray paint system is that it is not necessary to provide a special control mechanism for the motor, as the scan laser beams of the respective colors are linked with each other and scanned. A difference between these characteristics can be considered as the degree of freedom in positioning in the sub-scanning direction.
In the case of the four-series tandem system, there is a large amount of freedom in the timing of the positioning, as the motors are independent of each other. In other words, it becomes possible to position the colors at substantially the same position. It is possible to achieve this by controlling the rotation phases of the motors. On the other hand, in the case of the spray paint system, the positioning is carried out in the scan line unit, as the laser beams are based on one motor and thus related to each other. It is also possible to control the rotation phases in a unit smaller than the scan line unit based on other techniques. It has been known that according to a high-resolution image forming apparatus developed in recent years, a color deviation is not so noticeable even when the positioning is carried out in a scan line unit. In control based on a scan line unit using two laser beams, a maximum amount of deviation is one half line.
The color matching in the above-described color-image forming apparatuses will be described next. According to the tandem color-image forming apparatus, it is necessary to carry out the positioning by correcting the side registration, the lead registration, the scale factor, the scan line curve, and the scan line inclination. Among these items to be corrected, the correction of the lead registration that is a feature of the present invention will be described. Prior-art techniques will be described, with a detecting system and a control system being described in that order.
According to an apparatus described in JP-A No. 2-105169 as a first prior-art technique, for detecting a color deviation, at first, each photosensitive member is exposed and an image is developed using pattern images for measuring each color at a predetermined timing. Each pattern image of each color formed by this operation is transferred onto a transfer belt. Information on a position of color deviation is obtained by detecting position information of each pattern image with a reading sensor. According to this invention, a reflection-type phototransistor is used as a reading sensor. Therefore, in actual practice, a gap between pattern images of each color is obtained as color deviation information. It is possible to determine a deviation in the position of each color by measuring an output corresponding to the interval of the gap between the colors. Based on a result of the color deviation obtained in this way, the lead registration which is the write start position in the sub-scanning direction is corrected. There is also a similar method using a CCD sensor as a reading sensor.
In an apparatus described in JP-A No. 63-65457 which is a second prior-art technique, a reading sensor is disposed near a photosensitive member. Instead of detecting a pattern image as described above, an irradiation position of a laser beam itself is detected by a sensor like a PSD. By obtaining information about a magnitude of a deviation of this irradiation position from a correct irradiation position which should be scanned to avoid color deviation, the lead registration which is the write start position in the sub-scanning direction is corrected.
The control system will be described next. According to the tandem color-image forming apparatus, the precision of control in the sub-scanning direction is different depending on the type of the exposure device. In other words, as described above, in the case of the color-image forming apparatus of the four-series tandem system, independent scan rotation motors exist.
Therefore, the amount of freedom of control in the sub-scanning direction is high because the adjustment of the rotation phases is added. However, in principle, both types of exposure devices change the write start position in the sub-scanning direction by advancing or delaying the write starting time in the sub-scanning direction with respect to a predetermined reference time. Therefore, it can be said that the adjustment of the rotation phases in the four-series tandem system is a simple additional control for carrying out a higher-precision control. The principal control method can be used not only in the tandem color-image forming apparatus but also in the general black and white image forming apparatus. According to this control system, a control method in the scan line unit is used. Therefore, it is possible to provide a relatively simple structure based on a discrete lead registration control although it is not possible to increase the control precision. In the case of the four-series tandem system that has the rotation phase adjustment of the scan rotation motor added to the principal control method, it becomes possible to carry out a control in the scan line unit or in a smaller unit. However, the structure becomes complex in this case.
In the case of the tandem color-image forming apparatus, it is not possible to obtain a high-definition color image unless a color deviation is corrected based on one method and another. First, it is necessary to detect a deviation in color images. According to the above-described first prior-art technique, as a detection method, measurement pattern images for matching colors are formed on a transfer medium that is a medium for superimposing images. Then, these pattern images are read by the reading sensor, to thereby obtain a positional relationship between the colors, that is, a positional relationship between respective scanning laser beams. In this case, the high-definition color image means that the image has no color deviation. Further, the precision of the positioning (resolution) based on the color scanning laser beams is determined by the resolution of the reading sensor.
In the 600 DPI (Dots Per Inch) which has become a general write density in recent years, the resolution of at least a few dozens xcexcm or below is required for matching colors. It is very difficult to read a reflection beam of a toner image of four color patterns at this level of precision. In consideration of the need for a correction at a higher precision level based on a higher resolution, it is difficult to meet this requirement. In order to meet this requirement, it is essential to use an analog processing circuit to increase the precision in addition to increasing the sensitivity in each color area of the detection sensor. This results in an extremely expensive detection system. When a CCD sensor is used in the detector, it is clear that the system becomes more expensive. Therefore, as there are advances such that picture quality have higher resolution, it becomes more difficult to meet the requirements of the high precision detection system.
As a separate measure for improving the precision in the reading, special characteristics may be imparted to marks in the test image. In this case, it is necessary to carry out number of kinds of processing. This requires a heavy load on the processing system. In addition to the time required for the adjustment processing, more processing time is required. Further, when the processing system has high performance, the cost increases correspondingly.
As a method for detecting a position with high precision, there is a method of directly detecting a laser beam having directivity by a detector like a PSD based on the second prior-art technique. Particularly, in the case of detecting a focused laser beam, it is possible to obtain precision at a certain level. However, according to the second prior-art technique, there occurs a problem in the manufacturing and assembling of the color-image forming apparatus that includes an exposure apparatus. In other words, according to the prior-art technique, it has been necessary to determine in advance a reference position at which color deviation is not generated as the position that is to be scanned. As a result, in the prior art, it has been necessary to adjust this reference position at the time of manufacturing the exposure apparatus, and has been necessary to implement the adjustment when the exposure apparatus has been built into the color-image forming apparatus. At the time of the manufacturing and assembling, a slight deviation may occur due to error. However, as this deviation directly results in the color deviation in the color image, it is necessary to carry out this adjustment very precisely. Further, as it is necessary to compare the position information of the position to be scanned with the position information of the position currently being scanned, the post-processing system may become complex. Further, when the adjustment is made with high precision, the adjustment of the reference becomes burdensome, and only the comparative detection is not sufficient for the control. Thus, there has been a problem that it is very difficult to meet the strict requirement for higher precision to satisfy the demands that will be made for higher resolution.
Further, according to the second prior-art technique, only the positions which are to be scanned and are independently provided for each color are controlled. Based on this, the deviation of the lead registration necessary for matching colors of the color image is controlled. In principle, it is considered the amount of color deviation as a whole is minimized when color deviation is controlled based on relationship between the colors. However, in the above-described prior-art techniques, color deviation is controlled independently and for this reason, the precision of the correction has not been sufficient.
The present invention has been made in light of the above-described problems. It is, therefore, an object of the present invention to provide a color-image forming apparatus that can detect a position with high precision without using a special or expensive device in the detection system, that can correct a deviation of the lead registration which is necessary for matching colors of a color image in a relatively simple structure, and that can form a high-definition color image by correcting a color deviation into an optimum value below a scan line unit.
In order to achieve the above-mentioned object, a first aspect of the present invention is a color-image forming apparatus comprising: a plurality of photosensitive members; a light source for emitting a plurality of optical beams; a scanning apparatus for forming images of each color component on the photosensitive members by scanning the plurality of optical beams on the corresponding photosensitive members; a setter capable of arbitrarily and variably setting write start positions in sub-scanning directions of respective colors; a plurality of sub-scanning position detection sensors each for detecting a sub-scanning position of a scan optical beam of each color; and a calculating unit for calculating a write start position in the sub-scanning direction based on a set value of the setter and a detection result of each of the plurality of sub-scanning position detection sensors, wherein the color-image forming apparatus sequentially transfers the plurality of images formed on the plurality of photosensitive members onto a transfer unit such that the images are superimposed on the transfer unit, to thereby obtain a single image on the transfer unit.
According to the above aspect, the color-image forming apparatus has a setter capable of arbitrarily and variably setting write start positions in sub-scanning directions of respective colors, and a plurality of sub-scanning position detection sensors each for detecting a scan laser beam position of each color. A write start timing in a corresponding sub-scanning direction can be obtained by a calculating unit on the basis of set values, which are set arbitrarily and in advance by the setter such that the write start positions of the respective colors coincide, and a detection value obtained by the sub-scanning position detection sensor so that the lead registrations of respective colors have a predetermined relationship. When a scan beam position has changed, this change is reflected by a detection result of each of the plurality of sub-scanning position detection sensors. As a result, it is possible to obtain a write start timing in the corresponding sub-scanning direction by utilizing the detection result. Therefore, according to the above aspect, a desired value of the lead registration of each color is determined based on a set value set by the setter and a current position of the scan laser beam. Thus, it is not necessary to carry out a troublesome adjustment of the lead registration.
A plurality of the scanning apparatus, the setter, the sub-scanning position detection sensor and the calculating unit respectively may be provided for each color.
Further, according to a second aspect of the invention, there is provided a color-image forming apparatus of the above aspect, wherein the calculating unit calculates a write start position in the sub-scanning direction by adding or subtracting the set value of the setter and the detection result of the sub-scanning position detection sensor.
According to the above aspect, the calculating unit that calculates a write start position in the sub-scanning direction by adding or subtracting the set value of the setter and the detection result of the sub-scanning position detection sensor is a simple adder or a subtractor. Therefore, it is possible to simplify the control algorithm or shorten the processing time. Further, it is possible to provide the circuit in a small scale.
Further, according to a third aspect of the invention, there is provided a color-image forming apparatus of the above aspect, wherein the plurality of sub-scanning position detection sensors are provided within the scanning apparatus.
In principle, it is best to detect a scan beam position at a position near the photosensitive member on which an image is actually written by a scanning optical beam. However, the application of the present invention is effective when it is not possible to provide a sub-scanning position detection sensor within the color-image forming apparatus or when a compact color-image forming apparatus is desirable. Further, it is possible to concentrate a plurality of sub-scanning position detection sensors at one place by providing the sub-scanning position detection sensor within the scanning apparatus. Therefore, there is an advantage in that it is possible to form them on one substrate.
Further, according to a fourth aspect of the invention, there is provided a color-image forming apparatus of the above aspect, the color-image forming apparatus further comprising a temperature detection sensor for detecting a temperature within the color-image forming apparatus, wherein the calculating unit calculates a write start position in the sub-scanning direction at least prior to an image formation processing, and after a lapse of a predetermined time or when a temperature detected by the temperature detection sensor has exceeded a predetermined temperature.
The correction of a lead registration is carried out in order to form a high-definition color image. After the lead registration has been set once, a deviation of the lead registration is not so large. According to the above aspect, a write start position in the sub-scanning direction is calculated at least prior to an image formation processing, and thereafter, the write start position is calculated again after a lapse of a predetermined time or when a temperature has exceeded a predetermined temperature. Therefore, it is possible to prevent a further deviation in the lead registration. As a result, it is possible to maintain a high-definition color image.
Further, according to a fifth aspect of the invention, there is provided a color-image forming apparatus of the above aspect, wherein, in image formation, a write start position in the sub-scanning direction is determined during a period other than a period when the image is being formed.
When a lead registration is corrected during an image formation, a formed image may be disturbed by this correction. In other words, when the lead registration is corrected during the image formation, the state of a color is changed, lines are generated or the color becomes uneven. When in image formation, a write start position in the sub-scanning direction is determined during a period other than an image formation period as explained in the above aspect, it is possible to prevent a problem like a disturbance in the image. As explained above, when the lead registration is corrected only during a period other than the image formation period, these problems with the picture quality do not occur. Further, when the sub-scanning position detection sensor is provided at the outside of an image formation section, it is always possible to observe a positional variation in the scanning laser beam. Therefore, in the case of printing for each plurality of images, it is possible to correct the lead registration by utilizing the inter-image. It is also possible to shorten the correction time actually required. When the lead registration is corrected during the image formation, various high-voltage noises and impulse components of the image forming apparatus are generated. On the contrary, this adversely affects the control, and has a risk of disturbing the image. According to the above aspect, it is possible to prevent the correction of the lead registration from adversely affecting the picture quality.
Further, according to a sixth aspect of the invention, there is provided a color-image forming apparatus of the above aspect, wherein the plurality of sub-scanning position detection sensors have higher resolution than the image writing resolution and the color-image forming apparatus further comprises a converter for converting a detection result of each of the plurality of sub-scanning position detection sensors into a predetermined conversion value, and a calculator for calculating a positional deviation in the sub-scanning direction between images based on the conversion value obtained by the converter, wherein the conversion value is updated based on a result of the calculation by the calculator.
According to the above aspect, the resolution of the sub-scanning position detection sensor is higher than the writing resolution. Therefore, the position information of the scanning laser beam can be obtained in detail. Based on this information, it is possible to minimize the deviation in the lead registration in the case of the control in the same one scan unit. In other words, when the scanning laser beam is based on a scan line, it is possible to determine an accurate position, and carry out a proper control based on this position. As a result, according to the above aspect, the converter carries out a conversion so as to be able to compare the detection value of each color obtained by the sub-scanning position detection sensor in a value smaller than a scan line unit, thus enabling characteristic extraction. Due to the computer comparing the conversion values of the respective colors with each other and computing the sub-scanning direction positional deviations between the respective images and updating the conversion values on the basis of the results of computation, it is possible to minimize a deviation in the lead registration in the control based on the scan line unit.
As explained above, according to the above aspects of the present invention, there is a superior effect in that it is possible to carry out the setting and control of the write start position in the sub-scanning direction and to prevent a color deviation, at low cost in a simple structure.